Two-wire/four-wire converter

ABSTRACT

A communication system has a two-wire communications path carrying analog transmission and reception signals, and a four-wire communications path including a two-wire transmission path carrying digital transmission signals and a two-wire reception path carrying digital reception signals. A two-wire/four-wire converter for coupling the two-wire communications path to the four-wire communications path includes an analog/digital converter having an input coupled to the two-wire communications path and an output. A digital filter has an adjustable communications function for generating a predetermined terminating impedance for the two-wire transmission path. The digital filter has an input connected to the output of the analog/digital converter and an output connected to the two-wire reception path. An adder has one input connected to the two-wire reception path, another input connected to the two-wire transmission path and an output. A subtractor has one input connected to the output of the adder, another input coupled to the output of the analog/digital converter and an output. A digital/analog converter has an input coupled to the output of the substractor and an output coupled to the two-wire communications path. A resistor is connected in parallel with the input of the analog/digital converter.

The invention relates to a two-wire/four-wire converter for coupling atwo-wire communications path carrying analog transmission and receptionsignals, to a four-wire communications path including a two-wiretransmission path carrying digital transmission signals and a two-wirereception path carrying digital reception signals.

In two-wire/four-wire converters, the problem of impedance or apparentresistance adaptation between various lines of different communicationspaths is well known. For instance, incorrect adaptation causes poor echodamping properties or reflections, which markedly impair the quality ofcommunications. It is especially at a transition from a two-wirecommunications path to a four-wire communications path that a relativelyaccurate adaptation between the impedance of the two-wire communicationspath and a termination impedance formed by the two-wire/four-wireconverter is necessary. In that regard, the two-wire/four-wireconverters are often also constructed as an interface between analog anddigital signals.

In order to adapt the impedances of a two-wire communications path and atwo-wire/four-wire converter, passive components were originally used onthe analog side for adjusting the required terminating resistor. Thatmeant that additional components, most of them being relatively large insize, were necessary, and adaptation to different line impedancesentailed additional expense and effort.

For instance, German Patent DE 31 41 502 C2, corresponding to U.S. Pat.No. 4,381,561 discloses a two-wire/four-wire converter, in which theadaptation is carried out on the digital side by means of acorresponding digital filter. In that known two-wire/four-wireconverter, signals received on a two-wire communications path aresupplied to an analog/digital converter supplying a digital outputsignal which is further transmitted to a two-wire reception path. Theoutput signal of the analog/digital converter is added in a digitalsummation circuit to a signal applied to a two-wire transmission pathand delivered to a digital filter. The filter coefficients of thedigital filter are variable, for direct adjustment of a desired outputimpedance. The digital analog signal of the digital filter is convertedby a digital/analog converter into an analog signal that in turn issupplied to a voltage/current converter. The output of thevoltage/current converter is connected to the two-wire communicationspath.

In the known two-wire/four-wire converter, constructing thevoltage/current converter proves to be difficult. Relatively inexpensiveembodiments often have stability problems, while stabler embodimentsentail added expense.

It is accordingly an object of the invention to provide a two-wire/fourwire converter, which overcomes the hereinafore-mentioned disadvantagesof the heretofore-known devices of this general type.

With the foregoing and other objects in view there is provided, inaccordance with the invention, in a communication system having atwo-wire communications path carrying analog transmission and receptionsignals, and a four-wire communications path including a two-wiretransmission path carrying digital transmission signals and a two-wirereception path carrying digital reception signals, a two-wire/four-wireconverter for coupling the two-wire communications path to the four-wirecommunications path, comprising an analog/digital converter having aninput coupled to the two-wire communications path, and an output; adigital filter with an adjustable communications function for generatinga predetermined terminating impedance for the two-wire transmissionpath, the digital filter having an input connected to the output of theanalog/digital converter, and an output connected to the two-wirereception path; and adder having one input connected to the two-wirereception path, another input connected to the two-wire transmissionpath, and an output; a subtractor having one input connected to theoutput of the adder, another input coupled to the output of theanalog/digital converter, and an output; a digital/analog converterhaving an input coupled to the output of the subtractor, and an outputcoupled to the two-wire communications path; and a resistor connected inparallel with the input of the analog/digital converter.

In accordance with another feature of the invention, the two-wirecommunications path has a communications function, and thecommunications function of the digital filter includes a partialcommunications function being equal to the communications function ofthe two-wire communications path, multiplied by the inverse value of theresistance of the resistor.

In accordance with a further feature of the invention, thecommunications function of the digital filter includes a partialcommunications function exhibiting OFF-state behavior at a givenfrequency.

In accordance with an added feature of the invention, the two-wirecommunications path forms an impedance at a given frequency having avalue being greater than the resistance of the resistor by a certainfactor.

In accordance with an additional feature of the invention, there isprovided a transformer coupling the input of the analog/digitalconverter and the output of the digital/analog converter to one anotherand to the two-wire communications path.

In accordance with yet another feature of the invention, theanalog/digital converter has a raised sampling rate, and there isprovided a sampling rate reducing device connected downstream of theanalog/digital converter.

In accordance with yet a further feature of the invention, thedigital/analog converter has a raised sampling rate, and there isprovided a sampling rate raising device connected upstream of theanalog/digital converter.

In accordance with yet an added feature of the invention, the digitalfilter is a wave digital filter.

In accordance with a concomitant feature of the invention, the digitalfilter is an adaptive digital filter.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a two-wire/four-wire converter, it is nevertheless not intended to belimited to the details shown, since various modifications and structuralchanges may be made therein without departing from the spirit of theinvention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

FIG. 1 is a schematic and block circuit diagram of an embodiment of atwo-wire/four-wire converter according to the invention;

FIG. 2 is a substitute circuit diagram of a two-wire/four-wire converteraccording to the invention in the reception mode;

FIG. 3 is a substitute diagram of a two-wire/four-wire converteraccording to the invention in the transmission mode; and

FIG. 4 is a substitute circuit diagram of a two-wire/four-wire converteraccording to the invention when a message unit counting pulse istransmitted.

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is seen a two-wire/four-wireconverter according to the invention having an analog/digital converter,which includes a preliminary filter 4 with low-pass or bandpass behaviorand a following encoder 5. A resistor 13 is connected parallel to aninput of the preliminary filter 4. Connected to an output of the encoder5 which operates with oversampling, is a sampling rate reducing device6, which lowers the sampling rate of the encoder 5 by a given factor n.For instance, the sampling rate reducing device 6 may include a low-passfilter followed by a switch device that switches only through eachn^(th) value. A two-wire reception path 3 is connected to the output ofthe sampling rate reducing device 6.

The sampling rate reducing device 6 is followed by a digital filter 8having filter coefficients and thus communications functions which areadjustable in order to generate a predetermined terminating impedance orapparent resistance for a two-wire communications path 1. Thecommunication performance of the digital filter 8 can be expressed byits communications function HG, which in turn is composed of a partialcommunications function H and a partial communications function G. Anoutput of the digital filter 8 is connected to the two-wire receptionpath 3. The two-wire/four-wire converter according to the invention alsoincludes an adder 7 having one input which connected to the two-wirereception path 3 and another input which is connected to a two-wiretransmission path 2. An output of the adder 7 is carried to one input ofa subtractor 9 having another input which is connected to the output ofthe sampling rate reducing device 6. An output signal of the adder 7 isthus subtracted from the output signal of the sampling rate reducingdevice 6.

The subtractor 9 is followed by a sampling rate raising device 10, thatraises the sampling rate by a given factor, which in this case is againthe factor n. The sampling rate raising device 10 includes a repeaterdevice, which delivers a sampling rate n times in succession, as well asa following low-pass filter.

The sampling rate raising device 10 is followed by a digital/analogconverter, which includes an oversampling decoder 11 and an afterfilter12 with low-pass or bandpass behavior. An output of the afterfilter 12is connected to the input of the preliminary filter 4 through a windingof a transformer 14. The transformer 14 is provided for coupling thetwo-wire/four-wire converter to the symmetrically constructed two-wirecommunications path 1.

In order to explain the functioning of the two-wire/four-wire converteraccording to the invention, a distinction will be made below between twobasic modes, namely a reception mode in which a signal appearing on thetwo-wire communications path 1 is carried to the two-wire reception path3, and a transmission mode in which a signal appearing on the two-wiretransmission path 2 is carried to the two-wire communications path 1.For the remainder of the discussion, the transformer 14, the preliminaryfilter 4 and the afterfilter 12, the encoder 5, the decoder 11, thesampling rate raising device 10 and the sampling rate reducing device 6are assumed to be frequency-independent and collectively to have a gainof one.

In the reception mode, the result is a substitute circuit diagram asshown in FIG. 2. A signal source 16, which is assumed herein to beideal, emits a signal that is carried through the two wirecommunications path 1 to the two-wire/four-wire converter. Thecommunications performance of the two-wire communications path 1 and theinternal resistance of a non-ideal voltage source and the communicationsproperties of coupling elements, such as the transformer 14, can bedescribed by means of an impedance 15, which has a complex impedance Z.In the two-wire/four-wire converter, the signal emitted by the signalsource 16 is then carried both to the digital filter 8 and to thesubtractor 9, which subtracts the output signal of the digital filter 8from this signal. The resistor 13 is connected parallel to the input ofthe digital filter 8 and the subtractor 9. The output signal of thesubtractor 9 is then fed back to the two-wire communications path. Withoptimal adaptation, in other words with maximum power takeup, thevoltage U of the signal source 16 is twice as high as the voltage V atthe transfer point to the two-wire/four-wire converter. Assuming apartial communications function G=1, the following relationship isobtained, as a function of the voltage U, the complex resistance Z ofthe impedance 15, the resistance R of the resistor 13, the complexcommunications function GH of the digital filter 8, and the voltage V:

    U/V=(R H G+Z)/(R H G)=(R H+Z)/(R H).

The ratio U/V is then equal to exactly 2, if

    H=Z/R.

A substitute circuit diagram as shown in FIG. 3 results for thetransmission mode. A signal appearing on the two-wire transmission path2 and having a voltage W, is carried to one input of the adder 7, whilethe output of the adder 7 is connected to the subtractor 9. The outputsignal of the subtractor 9 is carried first through the impedance 15 andthen to the inputs of the digital filter and the subtractor 9. Theresistor 13 is connected parallel to the input of the digital filter 8and to the input of the subtractor 9. With optimal adaptation, thefollowing relationship exists for the voltage V at the transfer point ofthe two-wire communications path 1 and the two-wire/four-wire converter:

    W/x=-2.

The ratio W/x can be calculated as

    W/x=-(Z+R H G)/Z=-(Z+R H)/Z.

Once again, if H=R/Z, then V=-W/2. From this it then follows that thecommunications function H of the digital filter 8 equals thecommunications function of the two-wire communications path 1,multiplied by the inverse value of the resistance of the resistor 13.

In this connection it should be pointed out that wave digital filtersare particularly suitable as the digital filters 8, because they assurethe stability on the digital side under all circumstances. Adaptivefilters are also preferentially used, because they quickly enable themaximum possible adaptation, even to different impedances. Moreover,oversampling encoders and decoders are used in order to hold down thefilter expense on the analog side on one hand, and to increase theaccuracy of the two-wire/four-wire converters on the other hand.

In telephone communications technology, message unit counting pulses areoften transmitted to the telephone subscriber. With the aid of asuitable device, the applicable fees in a given case can then beascertained from the display or indicator. In order to transmit thesecounting pulses to the subscriber, alternating current pulses which arepredominantly used have a frequency which is either below or above thespeech band of 300 to 3400 Hz. The most frequently encounteredfrequencies that are used are 16 kHz, 12 kHz, and sometimes 50 Hz aswell.

Problems arise with counting pulses having a frequency above the speechband. Since counting pulses have to be transmitted from the exchangesystem to the subscriber over unloaded cables, and the frequencies abovethe audible range are therefore highly damped, the signal source voltagemust be quite high, if it is to drive the fee indicators on thesubscriber's telephone. Since the voltage must be dimensioned in such away that the fee indicator can operate satisfactorily even with themaximum allowable length of the connecting line, a very high voltage ispresent for telephone subscribers who are located in the immediatevicinity of their particular telephone exchange. As a result, thecounting pulses (frequencies of 12 or 16 kHz) may overdrive the inputstages of the analog/digital converters of the two-wire/four-wireconverters, such as the preliminary filter, and thus interfereconsiderably with data communications, if the counting pulses aretransmitted during the existing connection. For instance, blocks in theform of low-pass filters, which leave speech band signals unaffected anddamp the fee pulses, are therefore typically installed in datacommunications systems. However, additional analog filters entailconsiderable additional expense.

In the case where a message unit counting pulse is transmitted, thesubstitute circuit diagram of FIG. 2 applies in principle. However, inthis case the digital filter 8 then has a communications function HG, inwhich the partial communications function H is equal to R/Z, and thepartial communications function G is equal to one only in the speechband. In that case, the following equation applies:

    U/V=(G+1)/G.

Then if the partial communications function G is selected in such a waythat its value at a given frequency, namely at the frequency of the feepulse, is less than one, and is approximately equal to one atfrequencies in the speech band, or in other words if it exhibits OFFstate behavior at the given frequency, then the voltage of the messageunit counting pulses is reduced to G/G+1. With suitable sizing,overdriving of the analog/digital converter or of the preliminary filter4 is avoided.

Since the two-wire/four-wire converter requires a certain adjustmenttime because of transit times, the message unit counting pulses cannevertheless overdrive the analog/digital converter or the preliminaryfilter 4 within that time. In view of this, the resistance of theresistor 13 is selected in such a way that it is less than theresistance of the impedance 15 by a certain factor. In this case, theinternal resistance of the digital/analog converter or of theafterfilter 12 should be considered as somewhat on the order of a shortcircuit. In a substitute diagram which then results as shown in FIG. 4,a voltage divider including the impedance 15 and the resistor 13 isconnected to a signal source 17 for the message unit counting pulse. Theequation for the voltage V is then:

    V=R/(R+Z) U.

The ratio between the resistance of the resistor 13 and the resistanceof the impedance 15 is accordingly selected in such a way that thevoltage V does not overdrive the analog/digital converter or thepreliminary filter 4.

I claim:
 1. In a communication system having a two-wire communicationspath carrying analog transmission and reception signals, and a four-wirecommunications path including a two-wire transmission path carryingdigital transmission signals and a two-wire reception path carryingdigital reception signals,a two-wire/four-wire converter for couplingthe two-wire communications path to the four-wire communications path,comprising:an analog/digital converter having an input coupled to thetwo-wire communications path, and an output; a digital filter with anadjustable communications function for generating a predeterminedterminating impedance for the two-wire transmission path, said digitalfilter having an input connected to the output of said analog/digitalconverter, and an output connected to the two-wire reception path; anadder having one input connected to the two-wire reception path, anotherinput connected to the two-wire transmission path, and an output; asubtractor having one input connected to the output of said adder,another input coupled to the output of said analog/digital converter,and an output; a digital/analog converter having an input coupled to theoutput of said subtractor, and an output coupled to the two-wirecommunications path; and a resistor connected in parallel with the inputof said analog/digital converter.
 2. The two-wire/four-wire converteraccording to claim 1, wherein the two-wire communications path has acommunications function, and the communications function of said digitalfilter includes a partial communications function being equal to thecommunications function of the two-wire communications path, multipliedby the inverse value of the resistance of said resistor.
 3. Thetwo-wire/four-wire converter according to claim 1, wherein thecommunications function of said digital filter includes a partialcommunications function exhibiting OFF-state behavior at a givenfrequency.
 4. The two-wire/four-wire converter according to claim 1,wherein the two-wire communications path forms an impedance at a givenfrequency having a value being greater than the resistance of saidresistor by a certain factor.
 5. The two-wire/four-wire converteraccording to claim 1, including a transformer coupling the input of saidanalog/digital converter and the output of said digital/analog converterto one another and to the two-wire communications path.
 6. Thetwo-wire/four-wire converter according to claim 1, wherein saidanalog/digital converter has a raised sampling rate, and including asampling rate reducing device connected downstream of saidanalog/digital converter.
 7. The two-wire/four-wire converter accordingto claim 1, wherein the digital/analog converter has a raised samplingrate, and including a sampling rate raising device connected upstream ofsaid analog/digital converter.
 8. The two-wire/four-wire converteraccording to claim 1, wherein said digital filter is a wave digitalfilter.
 9. The two-wire/four-wire converter according to claim 1,wherein said digital filter is an adaptive digital filter.